prevent active log from being overwritten when agent starts

.changelog/11386.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+agent: Fixed an issue that could cause previous log lines to be overwritten
+```

command/agent/log_file.go

@@ -62,18 +62,23 @@ func (l *logFile) fileNamePattern() string {
 }
 
 func (l *logFile) openNew() error {
-	createTime := now()
 	newfilePath := filepath.Join(l.logPath, l.fileName)
-	// Try creating a file. We truncate the file because we are the only authority to write the logs
-	filePointer, err := os.OpenFile(newfilePath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0640)
+
+	// Try creating or opening the active log file. Since the active log file
+	// always has the same name, append log entries to prevent overwriting
+	// previous log data.
+	filePointer, err := os.OpenFile(newfilePath, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0640)
+	if err != nil {
+		return err
+	}
+	stat, err := filePointer.Stat()
 	if err != nil {
 		return err
 	}
 
 	l.FileInfo = filePointer
-	// New file, new bytes tracker, new creation time :)
-	l.LastCreated = createTime
-	l.BytesWritten = 0
+	l.BytesWritten = stat.Size()
+	l.LastCreated = l.createTime(stat)
 	return nil
 }
 

command/agent/log_file_bsd.go

@@ -0,0 +1,16 @@
+//go:build darwin || freebsd || netbsd || openbsd
+// +build darwin freebsd netbsd openbsd
+
+package agent
+
+import (
+	"os"
+	"syscall"
+	"time"
+)
+
+func (l *logFile) createTime(stat os.FileInfo) time.Time {
+	stat_t := stat.Sys().(*syscall.Stat_t)
+	createTime := stat_t.Ctimespec
+	return time.Unix(createTime.Sec, createTime.Nsec)
+}

command/agent/log_file_linux.go

@@ -0,0 +1,17 @@
+//go:build dragonfly || linux || solaris
+// +build dragonfly linux solaris
+
+package agent
+
+import (
+	"os"
+	"syscall"
+	"time"
+)
+
+func (l *logFile) createTime(stat os.FileInfo) time.Time {
+	stat_t := stat.Sys().(*syscall.Stat_t)
+	createTime := stat_t.Ctim
+	// Sec and Nsec are int32 in 32-bit architectures.
+	return time.Unix(int64(createTime.Sec), int64(createTime.Nsec)) //nolint:unconvert
+}

command/agent/log_file_test.go

@@ -50,13 +50,33 @@ func TestLogFile_openNew(t *testing.T) {
 	require.NoError(err)
 	defer os.Remove(tempDir)
 
-	logFile := logFile{fileName: testFileName, logPath: tempDir, duration: testDuration}
+	filt := LevelFilter()
+	filt.MinLevel = logutils.LogLevel("INFO")
+	logFile := logFile{
+		logFilter: filt,
+		fileName:  testFileName,
+		logPath:   tempDir,
+		MaxBytes:  testBytes,
+		duration:  24 * time.Hour,
+	}
 	require.NoError(logFile.openNew())
 
 	_, err = ioutil.ReadFile(logFile.FileInfo.Name())
 	require.NoError(err)
 
 	require.Equal(logFile.FileInfo.Name(), filepath.Join(tempDir, testFileName))
+
+	// Check if create time and bytes written are kept when opening the active
+	// log file again.
+	bytesWritten, err := logFile.Write([]byte("test"))
+	require.NoError(err)
+
+	time.Sleep(2 * time.Second)
+	require.NoError(logFile.openNew())
+
+	timeDelta := time.Now().Sub(logFile.LastCreated)
+	require.GreaterOrEqual(timeDelta, 2*time.Second)
+	require.Equal(logFile.BytesWritten, int64(bytesWritten))
 }
 
 func TestLogFile_byteRotation(t *testing.T) {

command/agent/log_file_windows.go

@@ -0,0 +1,14 @@
+package agent
+
+import (
+	"os"
+	"time"
+)
+
+func (l *logFile) createTime(stat os.FileInfo) time.Time {
+	// Use `ModTime` as an approximation if the exact create time is not
+	// available.
+	// On Windows, the file create time is not updated after the active log
+	// rotates, so use `ModTime` as an approximation as well.
+	return stat.ModTime()
+}